Blow molding of thermoplastics is a highly developed art involving generally forming a melt of a thermoplastic polymer, for example a polyolefin such as polyethylene or polypropylene, and extruding it as a tube in a vertically downward direction. The tube is extruded for a length sufficient to form a container, and a special mold is closed around the tube, pinching closed and cutting off both the top and bottom. A hollow needle is then inserted through one wall of the plastic tube and a gas under pressure applied. The pressure causes the soft, hot plastic to expand outwardly to conform to the mold shape. The mold is cooled sufficiently to permit the plastic to retain its shape, and thereafter the mold opens and the object removed. The cycle is then repeated.
Tandem molds for blowing two bottles simultaneously are shown in Dittman U.S. Pat. No. 3,457,590. However, this approach merely involves producing two bottles of the same size and shape in a single cycle. The molds are not designed to provide for multi-part containers having an inner container part spaced inwardly from an outer shell. Separate molds on separate machines would be needed. Further, for each defective part, two or more replacements would be blown at each pass, and thus with the Dittman type of tandem mold, it is more difficult to match and collate for assembly the required number of parts for multi-part containers.
Multi-part containers are shown for example in Piker U.S. Pat. Nos. 3,298,554; 3,214,830; 3,304,603; 3,132,759; 3,356,243; 3,313,438; and 3,467,562. All of these involve separately molding the inner container, removing it from a mold, and trimming it for assembly. The outer two-piece container shell is also blown separately in a different mold, and cut apart along special lines so a shell bottom snap fits into the top. The separately molded and cut parts are then assembled with foamed-in-place or fiberglass batt insulation.